The present invention relates to a method for forming a deployment criterion for means of restraint in a vehicle, where the vehicle acceleration in at least one direction is measured, the measured acceleration is integrated, the integrated acceleration is compared to a threshold that assumes a high value at low accelerations and a low value at high accelerations, and an exceeding of the threshold by the integrated acceleration is used as the deployment criterion.
A method of this type for forming a deployment criterion, for means of restraint is known from European Patent No. 458796. In the deployment algorithm described in this publication, the level of the threshold depends on the measured acceleration. Selecting a suitable threshold makes it possible to distinguish between deployment and nondeployment situations. Indeed, the deployment of means of restraint (airbags or seatbelt tensioners) should be prevented when the vehicle is involved in only a minor crash or, for example, when driving over railroad ties or the edge of a curb, thus preventing any risk of injury to the vehicle occupants. In the case of serious crashes, on the other hand, in which there is always a risk of injury to the vehicle occupants, nothing should prevent the means of restraint from deploying. Because there is often only a very narrow margin between crashes that require deployment of the means of restraint and those in which such deployment should be prevented, it is entirely possible that the wrong decision is made as to whether or not the means of restraint should be deployed.
The object of the present invention is therefore to provide a method forming a deployment criterion as reliably as possible that clearly distinguishes between nondeployment and deployment situations.
The object mentioned above is achieved in that the measured acceleration is subjected to low-pass filtration; the deployment threshold is formed as a function of the filtered acceleration signal. An acceleration signal is analyzed, as soon as it has been measured, in a first time segment to determine characteristic features indicating a crash event in which the means of restraint should not be deployed; and the cut-off frequency of the low-pass filter is reduced for a predetermined time segment if a nondeployment situation of this type is detected. Analyzing a measured acceleration signal in the beginning to determine whether features of a nondeployment crash are present, and influencing the deployment threshold accordingly, produces a much more accurate deployment criterion, which clearly distinguishes between deployment and nondeployment situations.
If a nondeployment crash were erroneously detected, although a serious crash requiring unconditional deployment of the means of restraint did indeed occur, the crash signals would be only slightly influenced by the brief reduction in the cut-off frequency of the low-pass filter.
It is expedient that the integration of the measured acceleration does not begin until the acceleration signal exceeds a predetermined signal level. Thus, very slight vehicle accelerations lying below a noise threshold are initially completely ignored when forming the deployment criterion.
The low-pass filter can preferably be an IIR or FIR filter whose parameters are varied upon detecting a nondeployment situation so that the filter cut-off frequency is reduced.
To detect nondeployment situations, the measured acceleration signal is advantageously integrated across multiple consecutive, short time windows in the first time segment, the acceleration signal integrated in this manner is then subjected to a threshold value decision, and a nondeployment situation is determined if the integrated acceleration signal initially drops below a lower threshold due to a vehicle deceleration and subsequently exceeds an upper threshold due to elastic vibrations in the vehicle body.